Article published in
Hot Mix Magazine, Volume 7, Number 1

SPECIAL PROCESSING REQUIREMENTS FOR MIXES WITH ASPHALT-RUBBER

It is quite accurate to say that asphalt-rubber mixes have been misunderstood in some areas of the hot-mix asphalt (HMA) industry. Or you could say it has been under-recognized. Or you might say it has been unfairly placed in a dark corner of the "experimental" shelf with other potential road-surfacing solutions.

In fact, in the mid-1990's, asphalt-rubber mixers (or "A-R" for short) gained a poor reputation because some producers used it in poorly designed mixes that ultimately failed. But today, proven formulas are being used successfully on the highways of several states. And there are HMA producers and DOT engineers in some states — around the world, for that matter — who believe A-R will play a major role in the future of HMA paving. They have the facts and figures to prove it, too. Here are a few examples:

• Roads paved with A-R mixes integrate the flexible properties of tire rubber into liquid asphalt cement (liquid AC), producing a binder material that is resistant to cracking. Experts say this can mean roads that last longer and require less maintenance.
• According to some industry observers, there are A-R roads in Arizona — where the process was pioneered — that have lasted more than 20 years with only minimal maintenance requirements.
• Some studies show that traffic noise can be reduced as much as 85% with A-R pavements.
• In some circumstances, a thinner mat can be used with A-R mixes. As a result, roads made with A-R require only about half the material that is needed for the traditional HMA pavements.
• A-R can also help reduce the mounds of discarded rubber tires that clutter landfills. For example, a 2-in. (5-cm) overlay of A-R hot-mix uses about 2,000 tires per lane mile. Currently, about 10 million tires are recycled each year in paving operations.

Despite all these potential benefits — and the fact that the process of belding rubber and AC has been around since the 1960's — A-R has been used extensively in only a few states: Arizon, California, Florida and Texas. There are several reasons for the slow development of this technology. One reason is that before 1992, the process of mixing rubber and liquid AC was patented and therefore controlled by the patent holder — a fact that tended to prevent wide usage. It also contributed to higher costs and less specification flexibility.

Perhaps one of the major factors that has slowed the development of A-R pavements in states other than Arizona, California, Florida, and Texas is the special processing requirements that go along with the ruber that is added to the liquid AC. But with the right equipment, the processing can be simplified. And the special needs of A-R mixes can be easily met — thereby putting HMA producers in a position to earn substantial profits in the future.

In general terms, the process for making A-R pavements is really quite simple. Liquid AC is added to a certain amount of recycled ground tire rubber (GTR) at an elevated temperature. The amount of GTR added is usually a little more than 15 percent of the total binder material. After blending, a reaction time of one hour at 350–375°F (175–190°C) is required to obtain the correct viscosity. The reacted A-R is then added to the aggregate to complete the mix.

The concept is simple. But of course, the details of handling the material make the process a little more complicated than that.

The first unique difficulty arises from the intrinsic nature of recycled tires. They ususally contain foreign material — specifically, small bits of steel belting and grit. These tough, grainy particles can cause excessive wear to the processing equipment. In addition, crumb rubber has a tendency to clump and can cause poor circulation. And any presence of moisture can cause foaming. These problems, however, can be solved with large-diameter, heavy-walled piping that reduces wear rates, and high, shear-type mixers that obtain a homogenous blend. Pumps should have special internal clearances that allow them to properly handle the foreign materials found in GTR, as well as the elevated operating temperatures.

Another obstacle in the production of A-R mixes is the temperature requirements for obtaining the desired chemical reaction. In the reaction chamber, the ambient crumb rubber will cause a 40°F (5°C) drop in temperature. This necesitates the introduction of virgin liquid AC into the mixing process at 425–450°F (220–230°C). Since normal liquid-AC storage is around 300–325°F (150–160°C), large hot-oil heaters and heat exchangers are needed to meet the heightened process-temperature demands. Plus, to meet the strict ratio requirements of liquid AC to crumb rubber, high-temperature volumetric compensating flow meters are required.

A third special requirement for A-R handling is the design of the reaction tank. When GTR, a solid, is suspended in a liquid, it has a tendency to separate. Because of this tendency, the reaction tank must provide a sufficient amount of agitation to prevent separation. The agitation device must keep the entire tank volume in a liquid state in order to prevent any build-up due to zones of insufficient flow. This "stirring effect" also provides an effective blending of the recycled GTR elements and natural ground-rubber constituents.

While A-R handling may require an investment in some specialized equipment, many asphalt producers are beginning to prepare their plants to process A-R.

For example, All American Asphalt, an HMA producer headquartered in Corona, California, recently entered the A-R market in that state. Company officials say tehy have already befun to realize the benefits of that move. All American serves southern California with two Astec Double Barrel hot-mix plants in Irvine and Anaheim, and three other facilities in Corona, Irwindale, and Westminster.

In recent years, the use of A-R in California's state paving projects has increased to the point where All American decided it was time to get into the A-R business.

"The use of rubberized asphalt has continued to increase in the state of California," said Daniel Stinson, asphalt plant manager for All American Asphalt. "We thought it would be necessary for us to obtain a rubber plant in order to stay competitive in the market."

For the processing equipment necessary to get into the A-R business, All American turned to CEI Enterprises, Inc., a member of the Astec Industries family of companies that specializes in the production of heating equipment, liquid-AC storage tanks, and A-R handling equipment. Stinson said the response the company got from CEI was very impressive.

"The people at CEI were very helpful in designing the plant according to what our needs were going to be," said Stinson. "For example, they could have tried to sell us something a little bit more elaborate or overextended, but they took the time to sit down with us and let us explain what we wanted. Then they came back and made suggestions on what would work best for us."

All American eventually purchased two rubber-blending plants from CEI, with the most recent addition delivered early in 2001. Both plants were equippe to handle the special requirements of A-R.

Blending of virgin asphalt cement with crumb rubber requires some highly accurate metering and flow control for all materials. Large hot-oil heaters are used to support the high temperature demands. Programmable logic controls are employed for control purposes, automating ratio blending recipes, and report generation.

 

Both plants were also designed to be completely portable. "By being able to move the rubber plants to all of our facilities," said Stinson, "we were able to cover a broad range of southern California."

In fact, Stinson said All American has made an art of moving its two CEI rubber plants around southern California: "We find that it's relatively easy to break down a plant, move it, set it up, and have it ready for production in a short period of time."

The mixture of blended ground tire rubber (GTR) and virgin asphalt is reacted in dual-compartment tanks to achieve the required binder properties. During production runs, one compartment of the tank is used for plant consumption, while the other is used to conatin the mixture as it goes through the reaction process.

 

Stinson said the company's move into the handling of A-R has been a positive experience. "Our monthly production rate has been fairly high, with a good portion of our A-R mix going into public agency jobs."

Stinson said that the transition to A-R handling has been a smooth and easy experience, especially since they had a helpful equipment manufacturer to guide them.

Pump skids provide the flow control to the hot-mix plant. For drum plants, a high-temperature volume-compensating meter is employed. In batch plants, a separate circuit is incorporated into the skid piping system for bypassing the flow meter.

 

"We've been very pleased with this A-R equipment," said Stinson. "The way CEI set up the plant, it is very easy to service the equipment. Everything is very accessible. Plus, we've had very little downtime."

Stinson said he is confident that All American got into the A-R business at about the right time.

"The rubber market in California started out slow," said Stinson, "but it has grown — both here and across the country. Increasingly, DOTs everywhere are looking for road designs that they wont' have to worry about for 15 or 20 years."

And is there any other advice that he would give to other producers?

"If someone is already an HMA producer, it would probably be a good idea for him to consider the rubberized-asphalt business."

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